On your way to the Tate Modern (after snagging some gourmet treats at the Borough Market) stop for a break at the Southbank Skate Park. Tucked under the Southbank Centre, the park is a graphiti covered maze of ledges and ramps. According to Wikipedia, the space has been used by skateboarders since the seventies, serving as the center of skateboard culture in London. Stand behind the railing so you don't get in the way of the tricks. Plenty of food and culture surround the skate park, including a book market and the nearby Millennium Bridge (Plus bonus views of Big Ben and Parliament.)